The skyline of San Francisco, California is framed by the north tower of the Golden Gate Bridge at sunset in this photo. California will experience unthinkable damage when the next powerful quake strikes, probably within 30 years, even though the state prides itself on being on the leading edge of earthquake science.

Robert Galbraith/Reuters

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BERKELEY, Calif.

Two small earthquakes hit the San Francisco area Thursday, jolting residents on the same day many Californians took part in an annual earthquake preparedness drill.

A quake with a preliminary magnitude of 3.8 struck in Berkeley at 8:16 p.m. PDT, the U.S. Geological Survey said.

The earlier earthquake struck at 2:31 p.m. and was centered across the bay from San Francisco, six miles beneath the University of California, Berkeley, campus, according to U.S. Geological Survey maps.

The second earthquake was about nine miles deep, and was centered just east of the campus, according to the USGS.

Jack Boatwright, a seismologist with the USGS, described the earlier quake it as a "sharp little earthquake."

Tami Humphrey, director of a preschool just north of Berkeley, was outside with her students when the quake struck.

"We felt it pretty good. It felt like a drop and then a shake," she said, adding that "the kids didn't even notice."

More than 8.5 million people signed up to participate in the preparedness drill, which took place at 10:20 a.m. and was labeled the Great California ShakeOut.

The quake also came almost 22 years to the day after the Loma Prieta earthquake struck the Bay area during the 1989 World Series. The magnitude-6.9 quake killed 63 people, injured almost 3,800 and caused up to $10 billion damage.

Seattle resident Joaquin Miller was in Oakland when Thursday's shaking began. He said he first thought it was coming from a passing big rig.